Need help identifing preamp circuit and DIY design suggestions

[magnarito]

Hi. I have an old PCB card, grabbed from some old radio studio equipment. Its from a box used buy the program director, to give cues to the sound engineer. This card would amplify a microphone  used by the director and send the signal to a speaker amplifier or in the control room. Unfortunately I do not have the schematics or pin-out for this card, but it looks quite simple and would be ok to figure out how it works. I have a little skill in electronics, but not enough to recognize all circuits. Here is a picture of the PCB. you can see the electric leads through the PCB.

My idea is to figure out the pinout on this card. Signal in, signal out and power, and some ideas how to add +48 Phantom power, a phase shift, and a input and output gain selects. I figure this preamp might sound nice, as it have a Neutrik NTM4 AG 1:4 mic input transformer, and an output transformer of unknown brand, since the sticker seems to be gone. The op-amps are TL072.

So far I have figured out that the uppermost pin (upper right in the picture),  would be +V since it goes to the + pin of the TL072's.
and the second pin minus. will ground or 0V be the 3rd that the two white caps are connected to?
Also the jumper in the bottom left. preset gain on the mic. back on the board its printed 40-70 db. So I would imagine, connecting a 6-pole switch here would alter the input or output on the signal, right?

hoping for some  good answers
Thanks in advance.
M

 

[gyraf]

Without going into detail, I'm pretty sure you have a FET-based ALC (automatic level control) in that circuit. That's probably what the blue trimmer is for.

Jakob E.